powerpc/ps3: Fix vuart sparse warnings

Fix sparse warnings like these:

  drivers/ps3/ps3-vuart.c: warning: symbol 'ps3_vuart_disable_interrupt_tx' was not declared. Should it be static?

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>